The large scale alloying of a solute metal with a molten host or solvent metal is generally dependent on the dissolution rate of the solute metal therein. This particularly the case where the solute metal has a melting point substantially in excess of the melting point of the host metal.
For example, manganese-aluminum alloys, which are widely used commercially, present the problem of alloying because of the high melting point of manganese which is 1245.degree. C compared to aluminum which has a melting point of about 660.degree. C. The direct dissolution of manganese (e.g. electrolytic manganese in particulate form) in molten aluminum is difficult in that the rate of dissolution is very slow, particularly since the temperature of the aluminum melt is maintained at from about 705.degree. to 760.degree. C, e.g. 730.degree. C to 745.degree. C, in a holding furnace prior to the casting of the aluminum into the desired shape. Commercial manganese-aluminum alloys generally contain up to about 1.5% by weight of manganese, although manganese contents of about 2% to 3% are known for special purposes (see U.S. Pat. No. 3,930,895).
One conventional method for adding manganese to aluminum is to prepare a master alloy containing by weight 4% to 8% manganese and the balance aluminum so that the melting point of the master alloy is as close as possible to the temperature of the aluminum bath to be alloyed so as to effect rapid dissolution of the manganese master alloy, whether added in the solid or molten state to the aluminum bath which is maintained between about 705.degree. C to 760.degree. C. A disadvantage of this method is that a large amount of master alloy is required to produce only a small increase in the manganese content of the final alloy.
A second method is to use a master alloy containing by weight about 30% manganese which is added in the form of pieces, platelets or other particulate form to permit rapid dissolution of the master alloy in the aluminum bath. This alloy has a melting point of about 960.degree. C which is much higher than the normal aluminum bath temperature of about 705.degree. C to 760.degree. C and, therefore, tends to dissolve slowly, even with agitation of the molten aluminum bath. Raising the temperature of the aluminum melt to accelerate the dissolution of the high manganese master alloy or even manganese itself is economically unfeasible since the temperature of the molten aluminum would have to be raised to well over 800.degree. C or 900.degree. C. Other prior art methods which have been proposed are as follows.
U.S. Pat. No. 3,592,637 discloses a method for the addition of a wide variety of alloying ingredients to molten aluminun comprising a blended mixture of 10% to 90% finely divided aluminum and 10% to 90% of the solute metals and alloys thereof, manganese being one of the metals. The addition of the blended mixture is said to accelerate the rate of dissolution of the solute metal.
In U.S. Pat. No. 3,788,839, finely divided metals are similarly used to effect alloying of metals. The method comprises using finely divided metals from the group Mn, Cr, Cu, Fe, Ni, Ti, V and Zr and alloys thereof containing at least 50% by weight of said metals and mixtures thereof. The finely divided metals (25% minus 80 mesh) are added to the molten bath in an amount by weight per unit area of the bath at least sufficient to penetrate the surface of the bath, the bath being stirred until the finely divided metal is completely incorporated therein.
Another method proposed comprised forming briquettes from a mixture of manganese and aluminum powder, the briquettes being then immersed in a bath of molten aluminum.
In U.S. Pat. No. 3,591,369, manganese is added to molten aluminum by providing the manganese with a coating of potassium chloride, such that, when the coated manganese is added to the bath, dissolution of the solid manganese metal is effected.
While the foregoing methods have been helpful in improving the dissolution rate of slowly soluble solute metals in molten metal baths, it would be desirable to provide an improved method for further accelerating the dissolution rate of slowly soluble solute metals. Examples of other solute metals are Fe, Ni, Co, Mn, Nb, Ta, V, Ti, Zr, Hf, W, Cr, Mo and Si.